Cyclophosphamide (CY) can augment cell-mediated immunity, including tumor-directed immunity, in experimental animals. Immunopotentiation is probably due to killing or reversible functional impairment of suppressor T cells. We have demonstrated that administration of CY 1000.mg/M2 three days before a sensitizing dose of keyhole limpet hemocyanin (KLH) significantly augments the acquisition of delayed-type hypersensitivity (DTH) to KLH in patients with advanced melanoma and colon carcinoma. We propose to determine whether pretreatment of such patients with lower doses of CY (100. and 250. mg/M2) augments DTH to the antigens, KLH, dinitrochlorobenzene (DNCB), and oxazolone, and the antibody response to KLH and S. adelaide flagellin. Furthermore, we will determine the cellular basis for the augmentation of immunity by studying the composition and function of blood mononuclear cells at various times after administration of CY. Composition will include enumeration of total T cells and the helper-inducer and suppressor-cytotoxic subsets by staining with fluoresceinated monoclonal antibodies and analyzing with the cell sorter. Function will include responses to mitogens and allogeneic cells (proliferation and generation of cytotoxicity) and generation of non-specific suppressor cells. Similar studies will be performed after incubation of mononuclear cells in vitro with activated CY (4-hydroperoxy-CY or plasma from CY-treated patients). Also, we will study the proliferative response of lymphocytes to KLH and DNCB-conjugated autologous mononuclear cells to determine whether these responses correlate with DTH, are augmented by incubation with actiated CY, and are specifically suppressed by T cells, or soluble factors made by T cells, from unresponsive patients. Finally, we will determine the effect of CY on tumor-directed immunity by vaccinating melanoma patients with autologous, irradiated melanoma cells mixed with BCG, either three days after CY pretreatment or without CY. We will compare the DTH responses to autologous tumor cells in vivo and the lymphoproliferative and lymphocytotoxic responses to autologous tumor cells in vitro of the CY and control groups.